Spherically symmetric association sites

Throughout this chapter association has been defined as being between two molecules that must be positioned and oriented correctly for association to occur. That is, both molecules participating in the association bond have directional association sites. Another common case would be an association interaction between two molecules where one has a directional association site, while the other has a spherically symmetric association site. This type of interaction could describe ion-water solvation or mixtures [42] of patchy and spherically synunet-ric colloids. 

Very recently Marshall and Chapman [90, 91] developed a new TPT to model mixtures of these types. Specifically, they considered a mixture of molecules with directional association sites d molecules) and molecules with a single spherically symmetric associations site (i molecules). The d molecules have CSW association sites as given in Eq. (4) with an association energy between d molecules... 

Going beyond the single site case, the theory was recently extended such that the d molecules can have an arbitrary number of association sites [90]. In this approach the interaction between s molecules was also that of the hard sphere reference fluid. To add spherically symmetric attractions (square well, U, etc.) between s molecules, one simply needs to employ the appropriate reference system (square well, LJ, etc.). Work is currently under way to employ this association theory as a model for ion-water solvation. 

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